For the United States, Article One, Section Eight of the Constitution says "Congress shall have power to ... declare War". However, that passage provides no specific format for what form legislation must have in order to be considered a "declaration of war" nor does the Constitution itself use this term. Many[who?] have postulated "Declaration(s) of War" must contain that phrase as or within the title. Others oppose that reasoning. In the courts, the United States Court of Appeals for the First Circuit, in Doe v. Bush, said: "[T]he text of the October Resolution itself spells out justifications for a war and frames itself as an 'authorization' of such a war."[1] in effect saying an authorization suffices for declaration and what some may view as a formal Congressional "Declaration of War" was not required by the Constitution.

This article will use the term "formal declaration of war" to mean Congressional legislation that uses the phrase "declaration of war" in the title. Elsewhere, this article will use the terms "authorized by Congress", "funded by Congress" or "undeclared war" to describe other such conflicts.

The United States has formally declared war against foreign nations five separate times, each upon prior request by the President of the United States. Four of those five declarations came after hostilities had begun.[2] James Madison reported that in the Federal Convention of 1787, the phrase "make war" was changed to "declare war" in order to leave to the Executive the power to repel sudden attacks but not to commence war without the explicit approval of Congress.[3] Debate continues as to the legal extent of the President's authority in this regard.

After Congress repealed the Gulf of Tonkin Resolution in January 1971 and President Richard Nixon continued to wage war in Vietnam, Congress passed the War Powers Resolution (Pub.L. 93–148) over the veto of Nixon in an attempt to rein in some of the president's claimed powers. The War Powers Resolution proscribes the only power of the president to wage war which is recognized by Congress.

The table below lists the five wars in which the United States has formally declared war against eleven foreign nations.[4] The only country against which the United States has declared war more than once is Germany, against which the United States has declared war twice (though a case could be made for Hungary as a successor state to Austria-Hungary).

Force withdrawn after six months. However, the Joint Resolution was likely used to authorize the Pancho Villa Expedition. In the Senate, "when word reached the Senate that the invasion had gone forward before the use-of-force resolution had been approved, Republicans reacted angrily" saying it was a violation of the Constitution, but eventually after the action had already started, a resolution was passed after the action to "justify" it since Senators did not think it was a declaration of war.[15][16]

On at least 125 occasions, the President has acted without prior express military authorization from Congress.[22] These include instances in which the United States fought in the Philippine-American War from 1898–1903, in Nicaragua in 1927, as well as the NATO bombing campaign of Yugoslavia in 1999.

The United States' longest war was fought between approximately 1840 and 1886 against the Apache Nation. During that entire 46-year period, there was never more than 90 days of peace.[citation needed]

The Indian Wars comprise at least 28 conflicts and engagements. These localized conflicts, with Native Americans, began with European colonists coming to North America, long before the establishment of the United States. For the purpose of this discussion, the Indian Wars are defined as conflicts with the United States of America. They begin as one front in the American Revolutionary War in 1775 and had concluded by 1918. The United States Army still maintains a campaign streamer for Pine Ridge 1890–1891 despite opposition from certain Native American groups.[23]

The American Civil War was not an international conflict under the laws of war, because the Confederate States of America was not a government that had been granted full diplomatic recognition as a sovereign nation by other sovereign states.[24][25] The CSA was recognized by the United States government as a belligerent power, a different status of recognition that authorized Confederate warships to visit non-U.S. ports. This recognition of the CSA's status as a belligerent power did not impose any duty upon the United States to recognize the sovereignty of the Confederacy, and the United States never did so.

In 1973, following the withdrawal of most American troops from the Vietnam War, a debate emerged about the extent of presidential power in deploying troops without a declaration of war. A compromise in the debate was reached with the War Powers Resolution. This act clearly defined how many soldiers could be deployed by the President of the United States and for how long. It also required formal reports by the President to Congress regarding the status of such deployments, and limited the total amount of time that American forces could be deployed without a formal declaration of war.

Although the constitutionality of the act has never been tested, it is usually followed, most notably during the Grenada Conflict, the Panamanian Conflict, the Somalia Conflict, the Persian Gulf War, and the Iraq War. The only exception was President Clinton's use of U.S. troops in the 78-day NATO air campaign against Yugoslavia during the Kosovo War.[citation needed] In all other cases, the President asserted the constitutional authority to commit troops without the necessity of Congressional approval, but in each case the President received Congressional authorization that satisfied the provisions of the War Powers Act.

On March 21, 2011, a number of lawmakers expressed concern that the decision of President Barack Obama to order the U.S. military to join in attacks of Libyan air defenses and government forces exceeded his constitutional authority because the decision was made to authorize the attack without Congressional permission.[26]

^Whereas the Government of Germany has formally declared war against the government and the people of the United States of America... the state of war between the United States and the Government of Germany which has thus been thrust upon the United States is hereby formally declared. The War Resolution